Some things my sister and BIL appreciated were sitting with her so she wasn't alone when he went out for shopping and other errands, picking up chemo meds from the pharmacy to save them waiting around at the hospital after her treatment (hospital pharmacies always took hours to sort out meds), make a selection of favourite music for the iPod, bring books or load up the Kindle with a selection for her, send late night texts to say you're thinking about her if she struggles to get to sleep, do her nails, massage her hands and arms with luxury creams and oils, give her a pedicure/reflexology, collect mementoes and print off articles or pictures of places and people that remind you both of happy times, write or find funny poems, knit her a cosy bed jacket, seek out recipes of meals she might enjoy, or bring a home-made meal for them, find information for them on the internet (Macmillan and NHS have lots of good pages - they might be too busy or tired whilst she's on chemo), help to keep friends informed of progress - the phone can be going constantly which can disturb rest. Offer to cut the grass or do a bit of weeding if the garden looks like it needs it. Offer to iron nighties or PJs, if she needs to lounge around in them. It all depends how she copes with her treatment. Offering to visit at a regular time they can depend on could be helpful. Anything to boost morale, help her keep cheerful.
Hope it goes well for them 